Support vectorisation with mixed vector sizes

Support vectorisation with mixed vector sizes

After previous patches, it's now possible to make the vectoriser
support multiple vector sizes in the same vector region, using
related_vector_mode to pick the right vector mode for a given
element mode.  No port yet takes advantage of this, but I have
a follow-on patch for AArch64.

This patch also seemed like a good opportunity to add some more dump
messages: one to make it clear which vector size/mode was being used
when analysis passed or failed, and another to say when we've decided
to skip a redundant vector size/mode.

2019-11-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* machmode.h (opt_machine_mode::operator==): New function.
	(opt_machine_mode::operator!=): Likewise.
	* tree-vectorizer.h (vec_info::vector_mode): Update comment.
	(get_related_vectype_for_scalar_type): Delete.
	(get_vectype_for_scalar_type_and_size): Declare.
	* tree-vect-slp.c (vect_slp_bb_region): Print dump messages to say
	whether analysis passed or failed, and with what vector modes.
	Use related_vector_mode to check whether trying a particular
	vector mode would be redundant with the autodetected mode,
	and print a dump message if we decide to skip it.
	* tree-vect-loop.c (vect_analyze_loop): Likewise.
	(vect_create_epilog_for_reduction): Use
	get_related_vectype_for_scalar_type instead of
	get_vectype_for_scalar_type_and_size.
	* tree-vect-stmts.c (get_vectype_for_scalar_type_and_size): Replace
	with...
	(get_related_vectype_for_scalar_type): ...this new function.
	Take a starting/"prevailing" vector mode rather than a vector size.
	Take an optional nunits argument, with the same meaning as for
	related_vector_mode.  Use related_vector_mode when not
	auto-detecting a mode, falling back to mode_for_vector if no
	target mode exists.
	(get_vectype_for_scalar_type): Update accordingly.
	(get_same_sized_vectype): Likewise.
	* tree-vectorizer.c (get_vec_alignment_for_array_type): Likewise.

From-SVN: r278240